Study of X-ray emission from the S147 nebula with SRG/eROSITA: X-ray imaging, spectral characterization, and a multiwavelength picture

Simeis 147 (S147, G180. 0-01. 7, ``Spaghetti nebula'') is a supernova remnant (SNR) extensively studied across the entire electromagnetic spectrum, from radio to giga-electronvolt gamma -rays, except in X-rays. Here, we report the first detection of significant X-ray emission from the entire SNR using data of the extended ROentgen Survey Imaging Telescope Array (eROSITA) onboard the Russian-German Spektrum Roentgen Gamma (SRG). The object is located at the Galactic anticenter, and its 3 size classifies it among the largest SNRs ever detected in X-rays. By employing sim 15 years of Fermi -LAT data, our study confirms the association of the remnant with a spatially coincident diffuse giga-electronvolt excess, namely 4FGL J0540. 3+2756e or FGES J0537. 6+2751. The X-ray emission is purely thermal, exhibiting strong O, Ne, and Mg lines; whereas it lacks heavier-Z elements. The emission is mainly confined to the 0. 5-1. 0 keV band; no significant emission is detected above 2. 0 keV. Both a collisional plasma model in equilibrium and a model of nonequilibrium collisional plasma can fit the total spectrum. While the equilibrium model -- though statistically disfavored -- cannot be excluded by X-ray fitting, only the absorption column of the nonequilibrium model is consistent with expectations derived from optical extinction data. Adopting an expansion in a homogeneous medium of typical interstellar medium (ISM) density, the general SNR properties are broadly consistent with an expansion model that yields an estimated age of yr, that is a rather old age. The preference for an X-ray-emitting plasma in nonequilibrium, however, adds to the observational evidence that favors a substantially younger age. In a companion paper, we explore an SNR-in-cavity scenario, resulting in a much younger age that alleviates some of the inconsistencies of the old-age scenario.